The present invention generally relates to the field of polishing. In particular, the present invention is directed to a polishing pad having grooves configured to enhance or promote mixing wakes during polishing.
In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting and dielectric materials are deposited onto and etched from a surface of a semiconductor wafer. Thin layers of these materials may be deposited using any of a number of deposition techniques. Deposition techniques common in modern wafer processing include physical vapor deposition (PVD), also known as sputtering, chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD) and electrochemical plating. Common etching techniques include wet and dry isotropic and anisotropic etching, among others.
As layers of materials are sequentially deposited and etched, the uppermost surface of the wafer becomes non-planar. Because subsequent semiconductor processing (e.g., photolithography) requires the wafer to have a flat surface, the wafer needs to be planarized. Planarization is useful for removing undesired surface topography as well as surface defects, such as rough surfaces, agglomerated materials, crystal lattice damage, scratches and contaminated layers or materials.
Chemical mechanical planarization, or chemical mechanical polishing (CMP), is a common technique used to planarize workpieces, such as semiconductor wafers. In conventional CMP using a dual-axis rotary polisher, a wafer carrier, or polishing head, is mounted on a carrier assembly. The polishing head holds the wafer and positions it in contact with a polishing layer of a polishing pad within the polisher. The polishing pad has a diameter greater than twice the diameter of the wafer being planarized. During polishing, each of the polishing pad and wafer is rotated about its respective center while the wafer is engaged with the polishing layer. The rotational axis of the wafer is offset relative to the rotational axis of the polishing pad by a distance greater than the radius of the wafer such that the rotation of the pad sweeps out a ring-shaped “wafer track” on the polishing layer of the pad. When the only movement of the wafer is rotational, the width of the wafer track is equal to the diameter of the wafer. However, in some dual-axis polishers, the wafer is oscillated in a plane perpendicular to its axis of rotation. In this case, the width of the wafer track is wider than the diameter of the wafer by an amount that accounts for the displacement due to the oscillation. The carrier assembly provides a controllable pressure between the wafer and polishing pad. During polishing, a slurry, or other polishing medium, is flowed onto the polishing pad and into the gap between the wafer and polishing layer. The wafer surface is polished and made planar by chemical and mechanical action of the polishing layer and slurry on the surface.
The interaction among polishing layers, polishing media and wafer surfaces during CMP is being increasingly studied in an effort to optimize polishing pad designs. Most of the polishing pad developments over the years have been empirical in nature. Much of the design of polishing surfaces, or layers, of polishing pads has focused on providing these layers with various patterns of voids and/or networks of grooves that are claimed to enhance slurry utilization and polishing uniformity. Over the years, quite a few different groove and void patterns and configurations have been implemented. Prior art groove patterns include radial, concentric circular, Cartesian grid and spiral, among others. Prior art groove configurations include configurations wherein the width and depth of all the grooves are uniform among all grooves and configurations wherein the width or depth of the grooves varies from one groove to another.
Some designers of rotational CMP pads have designed pads having groove configurations that include two or more groove configurations that change from one configuration to another based on one or more radial distances from the center of the pad. These pads are touted as providing superior performance in terms of polishing uniformity and slurry utilization, among other things. For example, in U.S. Pat. No. 6,520,847 to Osterheld et al., Osterheld et al. disclose several pads having three concentric ring-shaped regions, each containing a configuration of grooves that is different from the configurations of the other two regions. The configurations vary in different ways in different embodiments. Ways in which the configurations vary include variations in number, cross-sectional area, spacing and type of grooves.
Although pad designers have heretofore designed CMP pads that include two or more groove configurations that are different from one another in different zones of the polishing layer, these designs do not directly consider the effect of the groove configuration on mixing wakes that occur in the grooves. FIG. 1 shows a plot 10 of the ratio of new slurry to old slurry during polishing at an instant in time within the gap (represented by circular region 14) between a wafer (not shown) and a conventional rotary polishing pad 18 having circular grooves 22. For the purposes of this specification, “new slurry” may be considered slurry that is moving in the rotational direction of polishing pad 18, and “old slurry” may be considered slurry that has already participated in polishing and is being held within the gap by the rotation of the wafer.
In plot 10, new slurry region 26 essentially contains only new slurry and old slurry region 30 essentially contains only old slurry at an instant in time when polishing pad 18 is rotated in direction 34 and the wafer is rotated in direction 38. A mixing region 42 is formed in which new slurry and old slurry become mixed with one another so as to cause a concentration gradient (represented by region 42) between new slurry region 26 and old slurry region 30. Computational fluid dynamics simulations show that due to the rotation of the wafer, slurry immediately adjacent to the wafer may be driven in a direction other than the rotational direction 34 of the pad, whereas slurry somewhat removed from the wafer is held among “asperities” or roughness elements on the surface of polishing pad 18 and more strongly resists being driven in a direction other than direction 34. The effect of wafer rotation is most pronounced at circular grooves 22 at locations where the grooves are parallel, or nearly so, to rotational direction 38 of the wafer because the slurry in the grooves is not held among any asperities and is easily driven by wafer rotation along the length of circular grooves 22. The effect of wafer rotation is less pronounced in circular grooves 22 at locations where the grooves are transverse to rotational direction 38 of the wafer because the slurry can be driven only along the width of the groove within which it is otherwise confined.
Mixing wakes similar to mixing wakes 46 shown occur in groove patterns other than circular patterns, such as the groove patterns mentioned above. Like circular-grooved pad 18 of FIG. 1, in each of these alternative groove patterns, the mixing wakes are most pronounced in regions where the rotational direction of the wafer is most aligned with the grooves, or groove segments, as the case may be, of the pad. Mixing wakes are undesirable in many CMP applications because renewal of active chemical species and removal of heat are slower in the wake region than in the ungrooved areas of the pad immediately adjacent each groove. However, in other applications, mixing wakes can be beneficial precisely because they provide more gradual transitions from spent to fresh chemistry and from warmer to cooler zones of reaction. Without mixing wakes, these transitions can be unfavorably sharp and bring about significant variations in polish conditions point to point under the wafer. Consequently, there is a need for CMP polishing pad designs that are optimized, at least in part, based on the consideration of the occurrence of mixing wakes and the effects that such wakes have on polishing.